Call Transfer Scenarios in VoIP, SIP, IMS

UPDATED: 2014 October 1: A “Semi-consultative” call transfer cases are now included.

The IETF Call Control – Transfer draft seems to have the best and latest info on call transfer scenarios. But I haven’t find a good summary of the cases involving transfer-like scenarios.

— Blind transfer, original recipient is facilitator:
Alice calls Bob,
Bob answers,
Alice and Bob talk,
Bob transfers to Charles,
Charles answers,
Charles and Alice talk.
— Blind transfer, originator is facilitator:
Alice calls Bob,
Bob answers,
Alice and Bob talk,
Alice transfers to Charles,
Charles answers,
Charles and Bob talk.
— Blind transfer before call answer, original recipient is facilitator:
Alice calls Bob,
Bob’s phone rings,
Bob diverts the call to Charles,
Charles answers,
Charles and Alice talk.
— Blind transfer before call answer, originator is facilitator:
Alice calls Bob,
Bob’s phone rings,
Alice diverts the call to Charles,
Charles answers,
Charles and Bob talk.
— Attended transfer, original recipient is facilitator:
Alice calls Bob,
Bob answers,
Alice and Bob talk,
Bob puts Alice on hold,
Bob makes call to Charles
Charles answers,
Bob and Charles talk,
Bob completes transfer,
Charles and Alice talk.
— Semi-Attended transfer, original recipient is facilitator:
Alice calls Bob,
Bob answers,
Alice and Bob talk,
Bob puts Alice on hold,
Bob makes call to Charles
BEFORE Charles answers, Bob executes the transfer,
Charles Answers,
Charles and Alice talk.
— Attended transfer, originator is facilitator:
Alice calls Bob,
Bob answers,
Alice and Bob talk,
Alice puts Bob on hold,
Alice makes call to Charles
Charles answers,
Alice and Charles talk,
Alice completes transfer,
Charles and Bob talk.
—  Busy-Failed Blind transfer, original recipient is facilitator:
Alice calls Bob,
Bob answers,
Alice and Bob talk,
Bob transfers to Charles,
Charles is busy,
Alice and Bob talk.
— Busy-Failed Blind transfer, originator is facilitator:
Alice calls Bob,
Bob answers,
Alice and Bob talk,
Alice transfers to Charles,
Charles is busy,
Alice and Bob talk.
—  No-Answer-Failed Blind transfer, original recipient is facilitator:
Alice calls Bob,
Bob answers,
Alice and Bob talk,
Bob transfers to Charles,
Charles never answers,
Alice and Bob talk.
— No-Answer-Failed Blind transfer, originator is facilitator:
Alice calls Bob,
Bob answers,
Alice and Bob talk,
Alice transfers to Charles,
Charles never answers,
Alice and Bob talk.

Other Scenarios

Busy-Failed Attended Transfer

I don’t list the busy-failed attended call case, or the no-answer-failed attended call case, because those never get to the transfer (REFER message) case. So those just look like a call that gets put on hold, the facilitator makes another call, it fails, then the original call is retrieved from hold.

Semi-Consultative Attended Transfer

I only list one semi-attended transfer scenario. But in each case where an attended transfer may be possible, a semi-attended transfer is possible. This occurs when the facilitator starts a consultative transfer with the final recipient, but then transfers the call (e.g., SIP REFER) before the consultation part occurs. The term “Semi-Attended” transfer comes from BroadSoft Inc.